The present invention relates to a cadmium negative electrode for use in alkaline batteries such as a nickel-cadmium battery, and to a method for producing the same.
Cadmium negative electrodes for use in nickel-cadmium batteries known heretofore are classified into sintered negative electrodes and non-sintered negative electrodes. A sintered negative electrode is produced by filling a nickel-sintered substrate with a negative electrode active substance made of cadmium oxide or cadmium hydroxide. On the other hand, a non-sintered negative electrode is produced by first preparing a paste by kneading a negative electrode active substance comprising cadmium oxide or cadmium hydroxide with synthetic fibers, glue material, etc., and then coating the resulting paste to an electrically conductive core body (substrate) such as a punching metal and the like.
In case an alkaline storage battery using a cadmium negative electrode of the type above is charged and discharged, the product obtained at the cadmium negative electrode on charging changes into metallic cadmium, and that on discharging changes into cadmium hydroxide. In an alkali electrolyte of high concentration, which is generally used in an ordinary alkaline storage battery, the products obtained on discharging are crystal precipitates of hexagonal β-cadmium hydroxide (β-Cd(OH)2) that are stable in an alkaline electrolyte of high concentration. The β-cadmium hydroxide (β-Cd(OH)2) crystals possess thin platy hexagonal crystal structure, and even with a small amount, they may cause clogging at the apertures of fine pores of the cadmium negative electrode, or cover the surface of the active metallic cadmium.
If a case as such stated above occurs even in case non-discharged metallic cadmium (Cd) still remains inside the cadmium negative electrode, the non-discharged metallic cadmium finds difficulty in achieving ion conductivity with the electrolyte, and this makes discharging impossible. Furthermore, if the platy crystals of β-cadmium hydroxide (β-Cd(OH)2) as above grow into coarse grains, the surface of the particles decreases as to lower the rate of charge-discharge reaction. Then, difficulties occur on charging the coarse particles in the product on discharging, i.e., β-cadmium hydroxide (β-Cd(OH)2))
The coarse particles of β-cadmium hydroxide (β-Cd(OH)2) remaining without being charged further grow into coarser grains during discharging, and make the discharge of metallic cadmium (Cd) remaining inside the cadmium negative electrode still more difficult by clogging the numerous pores in the cadmium negative electrode or by covering the surface of the active metallic cadmium (Cd). In this manner, metallic cadmium (Cd) remaining non-discharged or cadmium hydroxide (Cd(OH)2) remaining non-charged tended to increase in amount with progressive charge-discharge reaction, and this gradually lowered the usability factor of the active substance to result in a problematic decrease in discharge capacity.
In the light of the aforementioned circumstances, there has been proposed, in JP-A-Sho61-158664 (the term “JP-A” as referred herein signifies “an unexamined published Japanese patent application”) or in JP-A-Sho61-158666, to form a coating of poly(vinyl alcohol) (PVA) or methyl cellulose (MC), which prevents the diffusion of hydoroxide ions, on the surface of the cadmium negative electrode or on the surface of the negative electrode active substance. In the cadmium negative electrode proposed in the published patent applications above, a coating of poly(vinyl alcohol) (PVA) or methyl cellulose (MC) formed on the surface of the cadmium negative electrode or on the negative electrode active substance functions as to hinder the diffusion of the hydroxide ions.
Since the concentration of the hydroxide ions decreases in the vicinity of the active substance during discharging, there grows γ-cadmium hydroxide (γ-Cd(OH)2), which is stable in an aqueous alkaline solution of high concentration. Since γ-cadmium hydroxide (γ-Cd(OH)2) generates in the form of monoclinic acicular single crystals, the surface of the metallic cadmium (Cd) is less covered by γ-cadmium hydroxide (γ-Cd(OH)2). Accordingly, not only the drop in usability factor of the active substance, but also the decrease in discharge capacity can be prevented from occurring as to suppress the degradation in charge-discharge cycle characteristics.
However, since poly(vinyl alcohol) (PVA) is sparingly soluble in water at room temperature, it must be dissolved in hot water in case of forming the coating on the surface of the cadmium negative electrode, and the resulting solution of poly(vinyl alcohol) (PVA) must then be applied or impregnated to the surface of the cadmium negative electrode. This caused a problem of making the production of cadmium negative electrode complicated. Furthermore, there was found another problem that a coating of poly(vinyl alcohol) (PVA) cannot sufficiently suppress the degradation of charge-discharge characteristics.
Further, in JP-A-Sho63-195963 is proposed a method of adding a polysaccharide, such as methyl cellulose (MC), or a derivative thereof into the cadmium negative electrode, and, although a polysaccharide such as methylcellulose (MC) certainly dissolves into an alkaline solution, it was unable to sufficiently exhibit the effect of suppressing the degradation. Hence, there occurred a problem that it insufficiently suppress the degradation of the charge-discharge characteristics. There was also proposed a method of adding poly (vinyl pyrrolidone) (PVP), which easily dissolves into water at room temperature, into the cadmium negative electrode. However, since poly(vinyl pyrrolidone) (PVP) swells in an alkaline solution, it could not sufficiently exhibit the effect of suppressing the degradation, and this also led to a problem of exhibiting insufficient effect on suppressing the degradation of the charge-discharge characteristics.